CIRCULATORY SYSTEM OF THE GRASS-SNAKE. 631 



B. The Posterior Azygos Vein (Y. Azygos posterior, Schlemm) 

 is a much shorter vessel than the anterior, and also than 

 the homologous vein in Lacerta. It originates at about 

 the level of the middle of the ventricle, and runs forward 

 to meet the anterior azygos just anterioi' to the right 

 auricle. In its course it receives three or four intercostal 

 veins, all of which come from the body- wall to the right of 

 the vertebral column. 



{h) Posterior Vessels. 



The Right Ptdmonary Vein (Y. pulmonalis, Schlemm) first 

 becomes noticeable as a definite vessel on the left side of the lung 

 at the level of the posterior end of the liver. It runs forward 

 along that organ to its anterior end, receiving numerous branches 

 and, leaving the lung, it runs parallel to the post-caval vein and 

 vential to the right systemic arch to open into the right auricle. 



A left pulmonary vein never exists at any time. 



The Caudal Vein (Y. caudalis, Schlemm) arises far back in the 

 tail and runs forward in the haemal canal, together with the caudal 

 artery. It leaves this canal and divides into two branches, the 

 renal portal veins, a short distance before reaching the cloaca. 



The Rencd Ported Vein (Y. venalis advehens, Schlemm) on each 

 side runs forward from the bifurcation of the caudal vein over 

 the cloaca and along the outer side of each kidney. It receives 

 soon after its origin a lateral tributaiy, which from its position 

 and distribution appears to correspond to a pelvic vein *. If this 

 be the case, it is interesting as being the only indication in the 

 circulatory system of the derivation of the snake from a limb- 

 bearing ancestry. Each renal portal vein receives several small 

 tributaries from the doi'sal body-wall, the cloaca, and the lateral 

 part of the tail. It then passes over the cloaca and lies between 

 the oviduct which is on its outer side and the ureter which is on 

 its inner side. Shortly after passing the cloaca each vein gives 

 off a well-marked branch, the right being anterior to the left, 

 which passes dorsally around the ureter and then runs ventrally 

 to unite with its fellow on the dorsal wall of the gut. The vein 

 so formed, the " Yeine mesenterique posterieure" of Jourdain 

 (27), runs forward along the gut and is continued as the hepatic 

 portal vein. In its course from the cloaca to the kidney each 

 renal portal vein I'eceives a number of small tributaries from both 

 ureter and oviduct, one of which, the Posterior Oviducal Vein, is 

 well marked, and also one or more from the dorsal parietes. On 

 reaching the kidney it runs along its lateral border closely accom- 

 panying the ureter, and, gradually getting smaller, disappears at 

 the anterior end of that organ. It is not continued anterior to 

 the kidney as in some snakes, e. g. the Boidee and also in Zamenis 

 gemonensis (Beddard, 4 & 2), a feature recalling the condition in 

 the Lacertilia. During its course along the posterior part of the 

 kidney this vein leceives a well-marked tributary from the 

 oviduct. 



* A similar pair of veins is to be found in Mmectes miirinus (Beddard, 2). 



